In travis-ci, build for osx also

Drop gcc b/c it takes too long to install via addon.

Build only static/release, to save VMs. (No shared to debug.)

travis.sh

@@ -17,6 +17,9 @@ set -vex
 
 env | sort
 
+echo ${CXX}
+${CXX} --version
+python3 --version
 meson --version
 ninja --version
 meson --buildtype ${BUILD_TYPE} --default-library ${LIB_TYPE} . build-${LIB_TYPE}